This invention relates generally to means for automatically controlling the operation of a heat pump system so as to manage and conserve the energy used by the system in meeting the room temperature requirements of the user with improved efficiency and at reduced operating costs. More specifically, the present invention relates to the use of a first microprocessor-based electronic circuit to form a user operated console for the manual entry therein of the user's room temperature requirements and a second microprocessor-based electronic circuit forming a heat pump functional system controller connected to the console for operating a heat pump to meet the user's requirements efficiently and at reduced operating costs as compared with manually controlled prior art heat pumps.
Heat pump systems are well known in the art and typically are provided in the heating mode with two or more states or levels of heating capacity. The first state normally involves only the operation of the heat pump compressor and associated refrigerant circuit to transfer heat energy from outdoor air or some other heat reservoir to the enclosed indoor space which is to be heated. In a single speed heat pump, changing to higher states or levels of heating capacity involves the use of one or more supplemental heat sources, such as electrical resistance strip heaters. In heat pumps having multi-speed compressors, additional states or levels of heating capacity can be provided by changing the compressor speed which changes the rate of heat transfer from outdoor space to the indoor enclosed space. In all such cases, the higher states of heating capacity are less efficient than the lower states, with the first state, i.e. compressor-only operation, being the most efficient. When operating in the cooling mode, variable states of operating capacity can only be provided by running the compressor at two or more operating speeds which changes the rate of heat exchange from the indoor enclosed space to the outdoor space.
Control of the operation of such prior art heat pumps is generally by means of an electromechanical form of a thermostat which includes a separate temperature sensor assembly and switching arrangement for each level of heating capacity. These temperature assemblies are usually mechanically interconnected such that a room temperature droop or undershoot must occur below the first stage operating point in order to energize the second stage and so on for the higher stages of heating capacity. Manual operation of this prior art thermostat in an attempt to increase system efficiency by reducing the setpoint, i.e. the desired room temperature setting, during periods of non-occupation of a building, for example, and subsequently increasing the setpoint during periods of occupancy creates an inefficient mode of operation while the heat pump is recovering from this setback of temperature. The reason for this is that this form of thermostat immediately increases the heat pump stage to its highest capacity in order to raise the room temperature and thus operates continually at its least efficient operating mode until the room temperature reaches the setpoint temperature.
Improved heat pumps have been developed which have variable heating and cooling compressor operating capacity. The intended benefit of such prior art variable capacity systems is, however, largely unrealized since it is extremely difficult for the user to determine whether the pump should be operated at a high or low compressor operating capacity in order to achieve the desired results in all circumstances. Moreover, a thermostat of the prior art type becomes complex and allows a significant temperature droop to occur. Furthermore, while high or low capacity heating or cooling may be required over a given period of time, short term changes in the numerous variables which affect heating and cooling rates can occur at any time in response to which a temporary change in the heating or cooling capacity of the heat pump would improve its efficiency and still permit user requirements to be met in a practical and reasonable time frame. The numerous variable factors which affect heating and cooling efficiency include not only the difference between the room temperature desired and the actual room temperature (the error temperature) but also the time rate of change of indoor temperature, and the outdoor temperature, just to mention a few. It is therefore not feasible to expect the operator of such prior art non-automatic variable capacity heat pumps to continually measure all of these factors and engage in the relatively complex computations from time to time that would clearly be required in order to operate these pumps at their most efficient capacities at all times.
The present invention substantially overcomes these and other difficulties encountered in the use of prior art heat pumps. It is therefore an object of the invention to provide an automatically controlled multi-state heat pump system and a method for controlling the energy usage of the system.
It is another object of the invention to provide a method of controlling a multi-state heat pump system and an energy management system therefor wherein the operating state, and consequently the energy usage, of the system is automatically controlled as a function of the rate of change of room temperature being controlled by the heat pump system.
It is also an object of the invention to provide a method of control and an energy management control system for a multi-state heat pump wherein energy usage of the heat pump system is controlled in accordance with the differential in temperature between the actual room temperature and the desired temperature setting to avoid excessive use of energy while attempting to reach the desired temperature within a reasonable time frame.
It is a further object of the invention to provide a heat pump system which is controlled, under certain operating conditions, to a target temperature which may differ from the user-inserted room temperature setting by a predetermined function of outdoor temperature in such a manner as to increase the perceived comfort offered by the heat pump system to the user.
Thus, in accordance with one of the aspects of the invention, there is provided a method of controlling a multi-state heat pump system to minimize energy usage while the system is being operated to bring the temperature of an enclosed space to a desired temperature level, which method comprises the steps of establishing a target temperature value representative of the desired temperature, sensing actual temperature of the enclosed space and initiating operation of the heat pump system in response to a differential between the actual and target temperatures. The method of this aspect of the invention further includes the steps of measuring the time rate of change in the actual temperature at the expiration of preselected time intervals during which an increase in operating state of the heat pump system to a higher level of energy usage is inhibited and increasing the heat pump system to the next higher operating state upon expiration of the preselected time intervals only when the time rate of change of actual temperature is less than a value required to substantially reach the target temperature within a predetermined time period.
In accordance with another aspect of the method of the invention, when a differential is sensed between the actual and target temperatures, the operation of the heat pump is initiated in the most energy efficient operating state needed to begin changing the actual temperature of the enclosed space toward the target temperature after which a further increase in operating state is inhibited for a fixed time interval during which the time rate of change is the actual temperature is measured. Then, in accordance with this aspect of the invention, a state change increase is initiated at the expiration of the inhibit time interval only when the rate of change in actual temperature fails to meet a predetermined criterion value which is in effect for the magnitude of the differential between actual and target temperatures existing at the time of expiration of the inhibit time interval, wherein the predetermined criterion value is representative of the rate of actual temperature change needed to reach the target temperature within a preselected time period, such as one hour from the first initiation of the heat pump system operation.
In accordance with a still further aspect of the invention, an energy usage management system is provided for a heat pump system having a plurality of operating states, each state representing a different level of energy usage wherein the energy management system comprises means for sensing actual temperature of the enclosed space and user-operated means for establishing a desired temperature setting for the enclosed space. The management system of the invention further includes means providing an operating state change criteria table comprised of a plurality of values of temperature differential between the actual and desired temperature levels at which operating state changes occur. The span of temperature differentials resulting in inter-state changes is preferably substantially greater, for example twice, than the span of temperature differentials resulting in intra-state changes. Inter-state differentials are those which occur when moving between adjacent operating states in the same direction as that of an immediately preceding state change, and intra-state changes are those which occur when moving between adjacent operating states in a direction opposite to that of the immediately preceding state change. The energy management system of the invention further includes means for comparing the sensed actual temperature to the desired temperature setting to determine the differential therebetween and means for comparing the temperature differential to the state change criteria table to derive a control effect representative of whether a particular operating state change is to occur. The system still further includes means responsive to the desired control effect for changing the operating state of the heat pump system to the particular state needed to bring the actual temperature to the desired temperature setting within a preselected time period.
In accordance with another aspect of the invention in the energy management system, means are provided for sensing the temperature outside the enclosed space and for adjusting the user inserted desired temperature setting upward as a predetermined function of the difference between outside temperature and the user inerted temperature setting to improve the perceived comfort offered to the user by the heat pump.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description and attached drawings upon which, by way of example, only a preferred embodiment of our invention is illustrated.